The present invention relates to housings for semiconductor devices and, more particularly, to housings for semiconductor devices having magnetic materials used therein which are to be protected from external magnetic fields.
A shield for protection from magnetic fields may be formed of a metal having a relatively high permeability. One such metal which is well known for use in magnetic shielding and has a high-initial permeability is known as Carpenter HiMu "80".RTM. alloy, having approximately 80% nickel, 4.2% molybdenum, 0.5% manganese, 0.35% silicon, 0.02% carbon and the balance of iron. This alloy is available from Carpenter I Technology Corporation, Carpenter Steel Division. Such alloys are referred to generally as Mu metal and are available from other sources.
Early efforts with DIP packages attempted to use a Mu metal shield that would serve as the lid as well as the magnetic shield. Mu metal lids with a gold tin sealing preform attached were solder reflowed for sealing. The temperature coefficient mismatch of the Mu metal and the DIP material caused the packages to crack under the temperature variation required for solder reflow. The use of a Kovar seal ring was simulated to see if the lid could be welded at room temperature. The simulation showed that the Kovar ring would couple a magnetic field into the package and therefore be a detriment.
U.S. Pat. No. 4,953,002 entitled "Semiconductor Device Housing with Magnetic Field Protection" dated Aug. 28, 1990 and assigned to Honeywell Inc. contains information on magnetic shielding and is hereby incorporated by reference.
For military, space or other applications requiring a high reliability package, it is desirable to have a hermetically sealed package that is free from any internal organic materials such as epoxy materials. In applications such as the radiation environment of space, it is also desirable to have the metal parts within the package at V.sub.SS or ground potential.